NAME

SYNOPSIS

DESCRIPTION

prctl() is called with a first argument describing what to do (with
values defined in <linux/prctl.h>), and further arguments with a
significance depending on the first one. The first argument can be:
PR_CAPBSET_READ (since Linux 2.6.25)
Return (as the function result) 1 if the capability specified in
arg2 is in the calling thread’s capability bounding set, or 0 if
it is not. (The capability constants are defined in
<linux/capability.h>.) The capability bounding set dictates
whether the process can receive the capability through a file’s
permitted capability set on a subsequent call to execve(2).
If the capability specified in arg2 is not valid, then the call
fails with the error EINVAL.
PR_CAPBSET_DROP (since Linux 2.6.25)
If the calling thread has the CAP_SETPCAP capability, then drop
the capability specified by arg2 from the calling thread’s
capability bounding set. Any children of the calling thread
will inherit the newly reduced bounding set.
The call fails with the error: EPERM if the calling thread does
not have the CAP_SETPCAP; EINVAL if arg2 does not represent a
valid capability; or EINVAL if file capabilities are not enabled
in the kernel, in which case bounding sets are not supported.
PR_SET_DUMPABLE (since Linux 2.3.20)
Set the state of the flag determining whether core dumps are
produced for this process upon delivery of a signal whose
default behavior is to produce a core dump. (Normally this flag
is set for a process by default, but it is cleared when a set-
user-ID or set-group-ID program is executed and also by various
system calls that manipulate process UIDs and GIDs). In kernels
up to and including 2.6.12, arg2 must be either 0 (process is
not dumpable) or 1 (process is dumpable). Between kernels
2.6.13 and 2.6.17, the value 2 was also permitted, which caused
any binary which normally would not be dumped to be dumped
readable by root only; for security reasons, this feature has
been removed. (See also the description of
/proc/sys/fs/suid_dumpable in proc(5).)
PR_GET_DUMPABLE (since Linux 2.3.20)
Return (as the function result) the current state of the calling
process’s dumpable flag.
PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
Set the endian-ness of the calling process to the value given in
arg2, which should be one of the following: PR_ENDIAN_BIG,
PR_ENDIAN_LITTLE, or PR_ENDIAN_PPC_LITTLE (PowerPC pseudo little
endian).
PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
Return the endian-ness of the calling process, in the location
pointed to by (int*)arg2.
PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
Set floating-point emulation control bits to arg2. Pass
PR_FPEMU_NOPRINT to silently emulate fp operations accesses, or
PR_FPEMU_SIGFPE to not emulate fp operations and send SIGFPE
instead.
PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
Return floating-point emulation control bits, in the location
pointed to by (int*)arg2.
PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
Set floating-point exception mode to arg2. Pass
PR_FP_EXC_SW_ENABLE to use FPEXC for FP exception enables,
PR_FP_EXC_DIV for floating-point divide by zero, PR_FP_EXC_OVF
for floating-point overflow, PR_FP_EXC_UND for floating-point
underflow, PR_FP_EXC_RES for floating-point inexact result,
PR_FP_EXC_INV for floating-point invalid operation,
PR_FP_EXC_DISABLED for FP exceptions disabled,
PR_FP_EXC_NONRECOV for async non-recoverable exception mode,
PR_FP_EXC_ASYNC for async recoverable exception mode,
PR_FP_EXC_PRECISE for precise exception mode.
PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
Return floating-point exception mode, in the location pointed to
by (int*)arg2.
PR_SET_KEEPCAPS (since Linux 2.2.18)
Set the state of the thread’s "keep capabilities" flag, which
determines whether the threads’s effective and permitted
capability sets are cleared when a change is made to the
threads’s user IDs such that the threads’s real UID, effective
UID, and saved set-user-ID all become non-zero when at least one
of them previously had the value 0. (By default, these
credential sets are cleared). arg2 must be either 0
(capabilities are cleared) or 1 (capabilities are kept). This
value will be reset to 0 on subsequent calls to execve(2).
PR_GET_KEEPCAPS (since Linux 2.2.18)
Return (as the function result) the current state of the calling
threads’s "keep capabilities" flag.
PR_SET_NAME (since Linux 2.6.9)
Set the process name for the calling process, using the value in
the location pointed to by (char*)arg2. The name can be up to
16 bytes long, and should be null terminated if it contains
fewer bytes.
PR_GET_NAME (since Linux 2.6.11)
Return the process name for the calling process, in the buffer
pointed to by (char*)arg2. The buffer should allow space for
up to 16 bytes; the returned string will be null terminated if
it is shorter than that.
PR_SET_PDEATHSIG (since Linux 2.1.57)
Set the parent process death signal of the calling process to
arg2 (either a signal value in the range 1..maxsig, or 0 to
clear). This is the signal that the calling process will get
when its parent dies. This value is cleared for the child of a
fork(2).
PR_GET_PDEATHSIG (since Linux 2.3.15)
Return the current value of the parent process death signal, in
the location pointed to by (int*)arg2.
PR_SET_SECCOMP (since Linux 2.6.23)
Set the secure computing mode for the calling thread. In the
current implementation, arg2 must be 1. After the secure
computing mode has been set to 1, the only system calls that the
thread is permitted to make are read(2), write(2), _exit(2), and
sigreturn(2). Other system calls result in the delivery of a
SIGKILL signal. Secure computing mode is useful for number-
crunching applications that may need to execute untrusted byte
code, perhaps obtained by reading from a pipe or socket. This
operation is only available if the kernel is configured with
CONFIG_SECCOMP enabled.
PR_GET_SECCOMP (since Linux 2.6.23)
Return the secure computing mode of the calling thread. Not
very useful for the current implementation (mode equals 1), but
may be useful for other possible future modes: if the caller is
not in secure computing mode, this operation returns 0; if the
caller is in secure computing mode, then the prctl() call will
cause a SIGKILL signal to be sent to the process. This
operation is only available if the kernel is configured with
CONFIG_SECCOMP enabled.
PR_SET_SECUREBITS (since Linux 2.6.26)
Set the "securebits" flags of the calling thread to the value
supplied in arg2. See capabilities(7).
PR_GET_SECUREBITS (since Linux 2.6.26)
Return (as the function result) the "securebits" flags of the
calling thread. See capabilities(7).
PR_SET_TIMING (since Linux 2.6.0-test4)
Set whether to use (normal, traditional) statistical process
timing or accurate timestamp based process timing, by passing
PR_TIMING_STATISTICAL or PR_TIMING_TIMESTAMP to arg2.
PR_TIMING_TIMESTAMP is not currently implemented (attempting to
set this mode will yield the error EINVAL).
PR_GET_TIMING (since Linux 2.6.0-test4)
Return (as the function result) which process timing method is
currently in use.
PR_SET_TSC (since Linux 2.6.26, x86 only)
Set the state of the flag determining whether the timestamp
counter can be read by the process. Pass PR_TSC_ENABLE to arg2
to allow it to be read, or PR_TSC_SIGSEGV to generate a SIGSEGV
when the process tries to read the timestamp counter.
PR_GET_TSC (since Linux 2.6.26, x86 only)
Return the state of the flag determining whether the timestamp
counter can be read, in the location pointed to by (int*)arg2.
PR_SET_UNALIGN
(Only on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15;
PowerPC, since Linux 2.6.18; Alpha, since Linux 2.6.22) Set
unaligned access control bits to arg2. Pass PR_UNALIGN_NOPRINT
to silently fix up unaligned user accesses, or PR_UNALIGN_SIGBUS
to generate SIGBUS on unaligned user access.
PR_GET_UNALIGN
(see PR_SET_UNALIGN for information on versions and
architectures) Return unaligned access control bits, in the
location pointed to by (int*)arg2.

RETURNVALUE

On success, PR_GET_DUMPABLE, PR_GET_KEEPCAPS, PR_CAPBSET_READ,
PR_GET_TIMING, PR_GET_SECUREBITS, and (if it returns) PR_GET_SECCOMP
return the non-negative values described above. All other option
values return 0 on success. On error, -1 is returned, and errno is set
appropriately.

ERRORS

EFAULTarg2 is an invalid address.
EINVAL The value of option is not recognized.
EINVALarg2 is not valid value for this option.
EINVALoption is PR_SET_SECCOMP or PR_SET_SECCOMP, and the kernel was
not configured with CONFIG_SECCOMP.
EPERMoption is PR_SET_SECUREBITS, and the caller does not have the
CAP_SETPCAP capability, or tried to unset a "locked" flag, or
tried to set a flag whose corresponding locked flag was set (see
capabilities(7)).
EPERMoption is PR_SET_KEEPCAPS, and the callers’s
SECURE_KEEP_CAPS_LOCKED flag is set (see capabilities(7)).
EPERMoption is PR_CAPBSET_DROP, and the caller does not have the
CAP_SETPCAP capability.

VERSIONS

The prctl() system call was introduced in Linux 2.1.57.

CONFORMINGTO

This call is Linux-specific. IRIX has a prctl() system call (also
introduced in Linux 2.1.44 as irix_prctl on the MIPS architecture),
with prototype
ptrdiff_tprctl(intoption,intarg2,intarg3);
and options to get the maximum number of processes per user, get the
maximum number of processors the calling process can use, find out
whether a specified process is currently blocked, get or set the
maximum stack size, etc.